Bad suggestion for `_` in `const` type

[estebank]

As noted in #81509, we currently emit the following suggestion, which is incorrect:

error[E0121]: the type placeholder `_` is not allowed within types on item signatures
  --> $DIR/typeck_type_placeholder_item_help.rs:13:22
   |
LL | const TEST4: fn() -> _ = 42;
   |                      ^
   |                      |
   |                      not allowed in type signatures
   |                      help: use type parameters instead: `T`

This can be solved by not suggesting type params ever, or at least when fn() types are involved.

[kper]

@rustbot claim